PHOTOTAXIS IN THE SEA URCHIN 133 



spines. On carefully observing such an animal from below 

 with a hand lens one can detect more or less working at cross 

 purposes among the tube feet and spines. While the majority 

 of the tube feet are very frequently directed away from the 

 light there are several which do not fall into line with the others 

 and whose failure to co-operate effectually checks the locomo- 

 tion of the animal. 



The direction in which the sea urchin moves is not deter- 

 mined simply by which motor elements exert themselves most 

 vigorously. Locomotion is a corporate activity in which the 

 various appendages co-operate much as they do in such animals 

 as centipedes or insects. Cutting the nerve ring destroys the 

 nervous connections through which co-ordinations are effected. 

 The result is that there is so much misdirected activity that 

 the sea urchin gets nowhere in particular. It may be that 

 other kinds of sea urchins may exhibit a certain amount of 

 phototaxis through the direct action of the parts most strongly 

 stimulated by light. From our observations on the light reac- 

 tions of the species experimented with such a result would 

 not be surprising. Specimens operated on when first exposed 

 to light would perform all the preliminary movements of the 

 spines and tube feet as if they were going to crawl away like 

 normal sea urchins ; but not one of them got further than making 

 a beginning. • 



GENERAL CONSIDERATIONS 



We have seen that in the sea urchin light sets up a series of 

 co-ordinated and definitely directed movements in relation to 

 the source of the stimulus. It also sets up definite reflexes of 

 particular organs of the sea urchin 's body. The question naturally 

 arises: How far can the light reactions of the animal as a whole 

 be regarded as the result of these reflexes? Light falling upon 

 the sides of the body causes the spines of those regions to make 

 movements toward the light. We may assume that it tends to 

 cause the tips to be depressed during their backward stroke, 

 so that each spine performs a sort of ro\ving action which tends 

 to push the body ahead, and there is a certain amount of evi- 

 dence for this in the fact that the spines on the side toward the 

 light are the most depressed while those on the opposite side 



